1. Field of the Invention
The present invention relates to a charged particle beam irradiation apparatus, and more particularly, to a charged particle beam irradiation apparatus having an accelerator.
2. Background Art
A charged particle beam (hereinafter referred to as an ion beam) which is ejected from a charged particle beam generation device such as an accelerator is a thin beam. The ion beam is enlarged (or scanned) in a plane perpendicular to the advancing direction of the ion beam by an irradiation field forming device in order to irradiate uniformly a cancerous tumor which is an irradiation target portion. The ion beam ejected from the charged particle beam generation device is in general a beam having a uniform energy. When this charged particle beam, particularly a proton beam and a heavy particle beam, are irradiated onto an irradiation target portion, a radiation dose distribution having a peak at a particular depth determined by their energies is formed. The peak of the radiation dose distribution is called Bragg peak. Since the spread of the Bragg peak is as narrow as several mms, the ion beam is enlarged by the irradiation field forming device so that the irradiation target portion is uniformly irradiated.
In addition, to irradiate uniformly the irradiation target portion with an ion beam, it is preferable that a plurality of ion beams having different energies be added together with their weights made mutually different. As described in Review of Scientific Instruments, Vol. 64, No. 8 (August 1993), pp. 2055-2122, there are known methods for enlarging an energy distribution, such as (1) a method of directly changing the energy of an ion beam from an accelerator (energy scanning) and (2) a method of passing an ion beam through a part of a rotating disk-shaped plate having an appropriately distributed thickness (range modulating propeller) as well as a method of disposing a wedge-shaped structure having an appropriately distributed thickness in an area through which an ion beam is to pass.
An energy distribution enlarging device using the energy scanning can impart a desired distribution to the total energy distribution of an ion beam to be irradiated onto an irradiation target portion, by appropriate control of an energy to be given to the ion beam from a radio-frequency accelerating cavity of an accelerator and by appropriate control of the amount of irradiation with the ion beam having the energy. An energy distribution enlarging device using the range modulating propeller (or the ridge filter) can impart a desired distribution to the energy distribution of a passing ion beam by setting to a desired distribution the distribution of the thickness of a portion through which an ion beam is to pass.
In addition, a method of widening an energy distribution to a Gaussian distribution in a narrow range compared to the spread of an irradiation target portion is described in Physics and Medical Biology, Vol. 44 (1999), pp. 2765-2775.
Japanese Patent Laid-Open No. 314324/1998 describes a ridge filter constructed by stacking a multiplicity of plates each having a width which becomes narrower toward its top. The ridge filter can be easily manufactured owing to such construction.
As the thickness of an irradiation target portion in the advancing direction of an ion beam becomes larger, the energy distribution required to irradiate the irradiation target portion with the ion beam becomes wider. In addition, as the depth of an irradiation target portion in the advancing direction of an ion beam becomes shallower, the spread of a Bragg peak becomes narrower, and therefore, the energy distribution of the ion beam needs to be made finer so that the ion beam can be uniformly irradiated onto the irradiation target portion. In either case, as described previously, if ion beams having different energies and weights are to be added together, a multiplicity of such ion beams need to be generated.
The ridge filter described in Japanese Patent Laid-Open No. 314324/1998 is capable of obtaining ion beam components having different energies by a number corresponding to the number of plates stacked in the advancing direction of the ion beam.
The invention aims to provide a charged particle beam irradiation apparatus capable of increasing the width in a depth direction of a Bragg peak to be formed, by means of a simple construction.
Therefore, according to the invention, a Bragg peak enlarging device for increasing the width in a depth direction of a Bragg peak formed by a charged particle beam scanned by a charged particle beam scanning device is provided with a first filter member and a second filter member which are disposed in order in an advancing direction of the charged particle beam and each of which has at least one thick-walled portion in a direction intersecting the advancing direction. The first filter member and the second filter member are disposed with their respective thick-walled portions intersecting mutually in the advancing direction of the charged particle beam.
Since the first filter member and the second filter member are disposed with their respective thick-walled portions intersecting mutually in the advancing direction of the charged particle beam, a plurality of charge particle beam components having different energies can be obtained by means of a simple structure. It is possible to increase to a further extent the width in a depth direction of a Bragg peak formed in the body of a patient by irradiation with a charged particle beam having such charge particle beam components. Such an increase in the width of the Bragg peak leads to a reduction in treatment time. In addition, in the case where a ridge filter is used, the ridge filter can be easily manufactured.
A first filter member and a second filter member which is provided with a thick-walled portion different in thickness in the advancing direction of a charged particle beam from the thickness of the thick-walled portion of the first filter member may be disposed with their respective thick-walled portions partly intersecting mutually in the advancing direction of the charged particle beam. In this case as well, the width in the depth direction of a Bragg peak to be formed can be increased by the above-described simple structure. In particular, the respective thick-walled portions of the first filter member and the second filter member are made mutually different in thickness in the advancing direction of the charged particle beam, and the first filter member and the second filter member are disposed with their respective thick-walled portions partly intersecting mutually in the advancing direction of the charged particle beam. Accordingly, it is possible to obtain charged particle beam components having different energies by a number larger than the number of the thick-walled portions different in thickness in the advancing direction of the charged particle beam. Accordingly, it is possible to obtain far more ion beam components having different energies by means of a simple construction in which fewer members are disposed in the advancing direction of the charged particle beam, whereby it is possible to increase the width of a Bragg peak in the depth direction.
Preferably, in the case where an affected part is divided into a plurality of irradiation target layers in its depth direction and a charged particle beam is irradiated onto each of the irradiation target layers by using a charged particle beam deflecting device or a charged particle beam enlarging device, an increase in the width of the Bragg peak can increase the thickness of each of the irradiation target layers and can decrease the number of energies to be changed.
Preferably, a Bragg peak enlarging device has a construction in which a plurality of filter elements are superposed on each other so that stick-shaped portions provided in the respective filter elements are arranged in mutually different directions, or a construction in which a plurality of filter elements are superposed on each other so that thick-walled portions provided in the respective filter elements are arranged in mutually different directions. This construction simplifies the construction of the Bragg peak enlarging device.